Presently the electrolytic production of titanium is performed in molten chloride systems and the metal produced has the form of pure crystals.
The industrial problem of chloride electrolysis is that titanium is deposited in the solid state on the cathodes with crystalline morphologies of large surface areas and low bulk densities.
The growth of the solid cathodic deposit requires its frequent removal from the electrolyte by means of handling apparatus of the kind described in U.S. Pat. No. 4,670,121.
The titanium deposit stripped from the cathodes retains some of the electrolyte entrained among the crystals, and the subsequent operation of removing the entrapped residual electrolyte, inevitably decreases the purity of the metal produced, which instead is very pure at the moment of its electrolytic reduction on the cathodes.
Also, the electrochemical characteristic of titanium deposition onto solid cathodes limits the maximum current density at which the electrolysis can be operated to relatively low values with correspondingly low specific plant productivity.
Further, in order to obtain crystalline deposits, the concentration of titanium ions in the electrolyte must be in the range requiring a separation between the anolyte and the catholyte as described in U.S. Pat. No. 5,015,342.
The electrolytic production of titanium in the liquid state has several operating advantages with respect to the production of solid deposits, as for example:
the cathodic area does not vary with the progress of the electrolysis, thus the achievement and control of steady-state operating conditions is easier; PA0 the separation of the pure metal produced from the electrolyte is complete and does not require any further operation besides solidification and cooling under a protecting atmosphere; PA0 the harvesting the metal produced can be performed without disturbing the progress of the electrolysis, as it will be explained in the description of the invention. PA0 the inherent refining capability of molten salt electrolytes which can maintain in solution some of the impurities or can separate others as vapor; PA0 some of the elements which are regarded as impurities by the pigment industry, are actually alloying metals for titanium alloys (for example: V, Zr, Al, Nb)
The electrolytic production of titanium at temperatures around its melting point has a very important thermochemical advantage, since the titanium lower valence compounds have a very low regime concentration, within the electrolyte, at those temperatures; therefore, there are no disproportionation or redox reactions to affect the current efficiency of the process (FIG. 9).
The electrolytic production of titanium at temperatures above its melting point has a very important electrochemical advantage, since the exchange current density values on liquid Ti cathodes are very much higher than those on solid Ti cathodes.
Furthermore, the addition of a minor ionic compound to the main electrolyte component, further increases the values of the exchange current density, since does not allow the formation of ionic metal complexes which are responsible for slowing the cathodic interphase processes.